Drill hole magnetometer apparatus



June 10, 1952 A. A. BRANT 2,599,687

DRILL HOLE MAGNETOMETER APPARATUS I June 10, 1952 A. A. BRANT DRILL HOLE MAGNETOMETER APPARATUS 2 Sl'lEETS-SI-IEET 2 3 FILTER E710/DRI( l I z FDWE/ SUPPLY Filed April 23, 1949 [rn/enfer fff'A.BraHlL l r my@ Mag?,

Patented `lune 10, 1952 Arthur A. ABrant,"Tm-ont physical signor to 0, ntarionanada, vas- Application/April v23, 1949, Serial No.1 89;335

' "In CanadaNovembei-il;1948

fiIli-isflmentmmrelatesto-ar-methed'fernunterlgra-fnean surveyingl -by :utilizing a novel magnel v tometer -deylce A.of the saturation induction .type "to be used down a drill holeto indicatevarations in the magnetic `f-fleldf sur-rounding` the -hole and hence fdetect the-presence of adjacent bodies 4having magnetic characteristics varying ffrom Y1-the normal l `,magnetic characteristics l of the 1o- M1cale.

The principal object of the invention is toaproavide a compact frugged f, and watertight magnetQmeter-detector unit, which :can be used down afdrlllv heleend` willvwithstand the lpressures `oi? water encountered down the hole and which-will tbeextremelysensitive.to indicate small variations `in the Amagnetic field intensity alongthetfhole.

A further important .object lis to provide a magnetometer in whichftlie-detectorf element.` will `loe;influenced only *byther vertical magnetic com- `1p onentdownf the 'drill hole independent,` of l theA inclination or angle-1o! Lthe hole and Awilletherefiore :fdenendablyI-record variations inthe mag- ,-.neticf:eld and `-not"fme1'elyrdiierent "lield com- :PQI'EDS- The principal feature of @the 1li-:invention consists-in provdingffa pressurefresista-nbwaterproof -.-nonmagnetic lcasing; adaptedtowbe inserted down a drillnoleand supporting a-,saturation induction detector unit or element within the casi-ngf'in'a noy l reby'tliegdetectorf.element'will nceofdgravityinmovement off the consista-:misus- .hln --a fvertically inea/ ns of a iflexible :suslice tlievrestoringfcouple s l d toslwing; liedetector:to-awertical-plane. still "iur-fibel'. -featui'e consists in providing :adjustable @countewbalancing means to enable the detector .elementtorbeaccuratelyabalanced to .1assume.a` trulyvertical position.

.Affurther .important featuregconsists inelormingwfthei detector unit as l an Veijltrelrrely compact single high Iimpedance coil wound ron a highly magnetic l core 'readily dri-vento saturation.

Figure 11 -is amid-vertical sectional View of Saturation. ivduqtenesteeter um` ,constructed in .faeordeaee with Yay mention and snowing the unitinsorted1 down@ .drill 1191s.

Figure-2 is an enlarged side elevational View .of ih? ,Qrinillflameassembly arranged -Within the delillf Film cfll (Cl. F75-182) Figure?xisfanfenlareed elcvationalrviewlofcthe saturationfdeteetor coil 4utilizing lmy ldetector `unit.

1ig.ll n block diagram,zillustratingthe Various l sireqllfiredto energize .theedetector 5 -unitfandi-to record-the:variations inthemmagnetic field detected by the detectorunit.

Eigurefaisaschematicffwiring diagram; of: the `con rol unit utilized myfdetectorllmt.

@laurea Saris. a diagrammatic viewy-il-lustmting ,3 lin the-presence v loffan externalmagnetic field andfillustratinghow the presence .oflaanaex- Atermal magnetic Aileld upsets fthe symmetry :of the voltage aware; form induced ,lnthe i-coll, ythe l5 illustrations corresponding to those obtainedmith @sinusoidal slgnalcurrentlbeingfapplied ,tothe 'OL .'Fgllre 5B illustrates .the u#wave form :of fthe E. M. F. induced in the Videtector.coil-byfthefllux n 2o fields sliowngin Figure 6A.

Figure 7 ,is :a parts sectional .view ithroueh .the kCasingand-.zcradicishovszinaa weight: secured to the cradle.

The use of ra saturationzinduction :magne- ;25 `tometer-unit as asensitive magnetic detectonhas *35 the f.- symmetry of, the rmagneticnlix @of ftlielnoil will be :unsetandtthe.resultant;V induced in a secondaryfcoilor back into the primarycoll itself,V `will have fav different characteristic when :theiiuxtof1-theccil istfcombined .f-withf-aneexternal `4.0 neldrthan.iitllelexternal neld were lacklnagam Ythis .challgecn the characteristicof -the induced M l?.canbefreadilydeterminedsto give :an indication l of the presence of .1 an external ismaeneitifield- Heretofore the `saturatcm induction :magnetometerhas been employed as` afsuriacel detection unit While such surface units l give a general indication of;V the ,magnetic characteristics 'lor rock or mineral formations adjacentv'the-:zearth surfacwthis information is ,Lnotsof fsumcient-faccuracy toj-permitoffa .comprehensive surueyliof subterranean formations to be made.

Owing to `theextremelydelicate.v nature ofthe .detectorfelement i particularly for: use Adami: dla.- ;-mond drill holesotmtype (1,15% .diameterirmnd 3 its necessity to be maintained aligned in the total magnetic field, it has not hithertofore been considered possible to use such a detector in making subterranean magnetic surveys.

The present invention however completely re- Vises the capabilities of the saturation induction magnetometer by providing a detector unit which, while ensuring all the sensitivity of a saturation induction element, can be inserted down a drill hole at any inclination.

With the devising of such a unit the present invention envisages a novel method of determining the magnetic characteristic of subterranean formations by sinking a drill hole through the formation to be expored and utilizing my saturation induction detector unit down the drill hole to obtain variations in the magnetic characteristics along the hole, which variations enable an accurate magnetic survey to be made.

In this manner valuable diagnostic information of the magnetic characteristics of a certain localer can be determined Without requiring the 'f necessity for sinking the numerous test drill holes l now required, thus greatly facilitating the survey of subterranean area and greatly reducing the time and cost involved.

Referring iirst to Figures l, 2 and 3 depicting my novel detector unit, it will be seen that the unit comprises a cylindrical casing I which is non-magnetic and preferably formed of stainlessy steel, and arranged within the casing is the orienting unit assembly comprising the frames 2 and 3 illustrated in Figure 2 with the frame 3 supporting the saturation induction element 4 shown in Figure 3.

Preferably the casing I is formed with a pointed nose and formed in the end wall of its bore 1 is a reduced axial bore 8 beyond which extends a central passage 9.

Mounted in the reduced axial bore 8 is a bushing I0 and in this bushing is a pivot pin I I springurged upwardly by the compression spring I2 arranged in the passage 9.

In spaced relation from the wall 6 of the casing bore is an annular shoulder I3 against which abutts a cylindrical insulating disc I4 in which is secured a bushing I5 similar to the bushing I0 and axially aligned therewith.

Mounted on the insulating disc I4 is a curved spring metal contact arm I6 which is adapted to maintain electrical connection with the bushing I5.

' The orienting assembly shown in Figure 2 y formed ofthe frames 2 and 3 is mounted be- V tween the bushings I0 and I5.

As shown the frame 2 is of generally rectangular configuration and is formed of an insulating material.

Extending laterally from each end of the frame 2 are substantially semi-circular projections i1 which give the frame the appearance of a cradle. Extending from the end of the frame or cradle 2 are pin projections I8 and I9 with the end of the upper pin projection I6 being pointed to provide a pivot point engaging the end wall of the upper bushing I5. The pin projection I9 projecting from the lower end of the cradle 2 is adapted to extend within the lower bushing I3 vand the end of said pin is engaged by the springurged pivot pin I I.

The cradle is thus supported to rotate about an axis coincident with the axis of the casing I with the major dimension of the cradle aligned with the major dimension of the casing, thus permitting a reduction of the casing diameter to permit it to be readily entered into the drill hole 20.

Secured to the semi-circular projections I1 of the cradle 2 are correspondingly shaped weights 2| which function to rotate the cradle about its longitudinal axis and maintain same in the desired oriented position with the weighted projections i1 downwardly where the casing is inclined at an angle to the vertical.

Mounted in the longitudinal side walls 22 of the cradle or frame 2 are bushings 23 which align with the transverse centre line of the frame. These bushings 23 form the bearing surfaces for the pin projections 24 of the frame 3 which is in the form of a cylinder or sleeve of insulating material.

The sleeve or frame 3 is thus supported to rotate about an axis transverse of the frame or cradle Zand its dimension is such that it can readily rotate within the cradle. r--'Encircling and secured to the sleeve 3 remote from the pivot axis is an annular or ring-shaped the influence of gravity, with the weight 25 always below the pin projections 24.

Secured to and extending over the end of the sleeve 3 remote from the Weighted member 25 is a curved metal clip 23, and suspended from this clip by means of a flexible suspension 21 in the form of a small metal chain is the saturation induction detector element shown in detail in Figure 3. This induction element comprises a coil 28 wound on a highly magnetic core 29 of Mumetal or other highly permeable magnetic material.

The upper end of the coil extends slightly above its core to receive a suitable plug 3i) to which the iexible suspension 21 is secured.

Secured to the periphery of the coil 2B in diametrically opposed relation are copper strips 3l which extend above the upper end of the coil and are bent outwardly forming balancing arms, the

. length and weight of which can be accurately controlled to ensure that the coil will assume a vertical position when held or suspended by the flexible chain or suspension 21.

Binding the strips 3| to the coil is a cellulose tape 32 around which is wound thread 33 with the thread and tape coated with a suitable bonding cement.

Connecting the ends of the coil 23 are leads in Y the form of pigtail windings 34 which permit the the threaded upper end of the casing and serves to lock theinsulating disc i4 against the annular shoulder I3. The cap 31 is formed with a bore 39 to receive the co-axial or twin conductor cable lill, with one of the conductors being grounded to the casing while the other is connected to the contact arm I6.

Surrounding the cable and threading into the outer end of the cap 31 is a cable tting or gland 6I which ensures a Water-tight iit between the cable and casing.

In using the detector device down a drill hole the cradle 2 carrying the weights 2I automatically surroundingthedr un revit' so tha izon- .at The s ,becomes ,omen

Qi gravity nit-ticalei @ier elementi# ihe-.fvrm-1fif'ti1s suppe te e within the sleeve by means 2 freely swing nel? istilly; baliss bfy ,ihiflvls 4.Will beilllllil by ille f there! 28 i9 Swiss es @Peris-111.14m i0 VIn V utilizing the satura-tion induction element in the form of the coil 28 a signal is impressed down the oahle 4 il, as will hereinafter be described,l and. this signal will be applied between the --grounded casing which connects through the lower bushing I0 to oneside of the coil 28 across the coil, the opposite end of which Vconnects through the upper bushing 4l 5 and `contact arm i6.

51.119, 4Casing l and the enclosed orientating assemblies and saturation induction detector element form a pressure-resistant Water-tight unit which can be `readily utilized down a drill hole .and vil/hich will .ensure that, independent Vof the inclination'of .the hole" orfoposition `of the casing, `thecoilf28, whichiorrns the saturation induction .detector element,.will `automaticalily and accurately orient iniitheyertical planeto detect Ionly the vertical compon .mbsf .the wiel megnetield ill hole. lntheenergization or the saturation induction detection .element in the form of the coil 28 wound on .the highly permeable core 29, a signal ,generator or oscillaterA 42 capable` ofdelivering a sinusoidal current' waveform.A output, .preferably of a lowifreouency, say-for convenience, a thou- ,sand cycles `.per second, is connected through a .controlunit43, asshown in Figurehto energize the. detection element 237-29.

.Since the @Parrains may flqllfeltly be em' ply there 11j@ SsrC 0f electrical weer 1S available, the equipment is run vfrom .a fifvolt orage battery 44 in conjunctionwitha vibratory .power supply unit 4 5.

'The .control nnit`43 includes aiilter section 45 (see Figure ,55 anda D. Cfsection forbucking `out the'ambient earthsmagnetic field linked with the Adetector coill The iilter section`4$ consists of a D.`C.f,blocking condenser F1 anda 200i) cycle blcckingglter system yidentiiifedQ generally by F2 andra F2 binf" timed sharply@ `1000 Eyelet ,Cnsequently this lter section permits only the V.any constant ,or ambientmagnetic eldacting ,uponthefdetector .coil 28. A2000 cycle response from thecoil 28 produces a 2000 cycle voltage serres ieersssir ,5.2L which voltage isimsrslssd @baths isses .ressa-i4. @were e) @este `is@ .Sine were signal, sunset illustrated bythe -Sieiloieel' vferri .48 produsse vite the l @been esfansxisrriel .Isa

neueste f1 eveny clipped te,fom` 'theV symmetrical linx form having` clipped peaks as illustratedbythe Vsolid straightyline peaked Wave forInLllS.M

The een eax iuustiateu by mecum 4 9, (Figurs 6A) y leads to th@ symmetrical ,induces E.- Waveform 50 in the'coil 28, (Figure 6B) Asthis Waveform is symmetricalit gives rise, as is `well known by Fourier and other analysis, to only, odd order harmonics, and thus the E. M. Rinduc'ed back into the coil 2,8 Will include only `the iirslt. third, fifth, etc. harmonics.

Connected through the control unit via the terminal 5I, variable resistance contact 52and test switch 53 is a receiver unit 54, Figure 4,which has a high sensitivity and which is tuned `with high selectivity to a 200G-cycle signal end serves to indicateor detect signals arriving b ack `from the coil 2 8. .since the receiver is tuned to only the Z900-cycle component ofany signal the receiver will'not give any indication of the presence of the third, fifth,` etcfharnionics of the 100i)- cycle signal and, therefore',l does not respond to a coil fluit changesuch as thatfshown in the curive 49 of Figure ,6i-3., or to an induced coill E. M. F. such as thatshown by the curve 50 of Figure 6B.

Where the saturation induction coil 218 is located in a magnetic eld the flux waveform surrounding'thecoil willbe distorted from the symmetrical fluit waveform 49 of Figure 6A Vi'.o"'the clippednonlsymmetrical fluir waveform 5 5"lia'v ii'ig th dotted Astraight-line peaks, andV thisnonsymmetrical flux waveform 55 gives rise `tothe nonsyrnmetrical induced E. M. F. Waveformr56, Figure. 6B which in srsbraisls a percentage 0f evenharinonics o f the IOOQfcycle sinusoidal eurrsnl Signal- These second harmonics are acceptedby the tuned receiver 54 which amplies the 2 0ll0gcycle `second `harmonic suciently so that itV may be readily., detectediby alineter or earphones.

Since the normal earth magnetic eld creates a second harmonic of the signal voltage, I prefer to balance the secondi harmonic for zero output in the `receiver at alchosen reference station.. rThis balancing is accomplished by utilizing a direct current to balance, or buck out, the effects of the magnetic field atthe detector coil 28. The magnitude of this direct current passed through the caille is adjusted until the ux wave form hallg@ ,fl'ilmt'he Symmetlcal UWV@ 55 (Figure 6A) to the symmetrical curi/e149', at which point any 2000 cycle component in the tuned receiyer 5.4 S?-TPP?P1S Til @alarming i5 aicvlpllshed byntilising a iixedvoitagesource 57 in the form .oi a plurality'oi mercury cells and utilizing-wi`th the lxed `voltage source the series decade resistances 58 which may be adjusted as desired, and included in or out of the circuit ley-means of isser-item3 er@ 'er initier essere tre? l A meterV 63, can. be connected, selectively, through the. switchV 5 3 to check variation in the 'de-magnetizing D. C. current, to check theamplitilde of the 100G-cycle component sinusoidal A. C.

current and to calibrate the apparatus in relation to the detector element. j

Thus at a reference station the resistors 5t and switch 59 are adjusted to give zero signal in the receiver V5l! and they are again adjusted at a new station with the adjustments calibrated to indioate the variation in the magnetic field from the reference to the new station.

The electrical circuits of the various units in` Yclude of course the necessary adjustments and variables to ensure their proper operation and it is deemed that the block showing of Figure 4 amply illustrates the manner in which my detec-f tion magnetoirletei unit is utilized as a detection element to indicate vat areinote station the varian tions in the magnetic characteristics of the rocl;

formationadjacent the hole at various stations doyvrl the drill hole.

In opration my detector unit enclosed iii the casing I is inserteda desired distance down the lil and is allowed to remain stationary until the pendulum motinof the coil 28 has ceased and the D. C. voltage required to balance the external field in give zero outputin the receiver unit is measured bythe meter 63 and this reading com pared with the reading required to balance the 1 dei@ at a reference Station.

The readings taken at various Apoints down the hole will thus give Valuable diagnostic informa tion in producing a magnetic survey in terms ci the vertical component of the magnetic i'leld.

With my development of a magnetometer de vtection unit which can be used down a drill hole and will orient with accuracy to measure the ver-l tical component of the external field, I have introduced a new and extremely important method of obtaining comprehensive subterranean magH netic surveys, and it will be readily appreciated that my method of utilizing a saturation induction detector at various subterranean points will greatly facilitate magnetic surveying and will materially reduce the time involved in conducting such surveys.

ing samples of the magnetic material adjacent ff the locale further adds to the importance of my method wherein a single drill hole enables a dem termination ci the magneticv characteristics throughout the locale to be determined.

As the detecting coil 2e maintains an aligned vertical position in the total magnetic field the variations in the second harmonic signal output from the coil will be accurate indications of variations in the vertical field component so that positive information is continually conveyed to the surface station.

While I have preferably used the single coil saturation induction element to reduce its size to a dimension which will permit it to he mounted in a casing that can :be inserted down a drill hole.

It will be'appreoiated that variations in the detection element may be made without departing from the scopeoi my invention. While the coil 2S is permitted free swinging movement and its inner supporting sleeve frame 3 can freely swing as permitted by the pig-tail vvwindings 3s, to prevent the continuous rotation of the sleeve to a point where the pig-tail windings become wound around the pins 2li, suitable stops 64 are incorporated on the cradle frame 2 8 to prevent complete rotation of the sleeve frame onitspivots. l

As previously emphasized, to obtaina required degree of accuracy the saturation induction ele'- ment must be maintained with its axis vertical and must have as great a length as possible within the casing. It has been established that to give a desired accuracy of 5 gammas the induction unit must be maintained in a vertical position within approximately .003 radian. It will be appreciated that normal gimbal suspensions will not provide this desired accuracy due to friction at the pivots, but such accuracy can be obtained with my pendulum suspension within a roughly orienting gimbal arrangement.

While preferably I measure vertical ii'eld ini tensities it will be appreciated that the satura.- tion induction element may be mountedon a suspension to assure a horizontal position to measure' the horizontal magnetic eld component, and it will be understood that such arrangements are fully contemplated within the 'scope of rriy'inver`1` tion.

In view of the foregoing, it will he appreciated that the development cf my novel maghetorreter apparatus which leads to a new method of drill hole magnetic surveying will be extremely i1riportant in mining and similar operations, the present unit readily permitting the recording of the vertical (or horizontal) magnetic intensity in drill holes of any inclination and down to the smallest size.

XV hat I claim as my invention is;

l. In a drill hole magnetcmeter apparatus, a longitudinal cylindrical non-magnetic waterproof casing, an eccentrically-weighted frame pivoted Within said casing to rotate vabout an axis coinciding with the axis of said casing, a second eccentrically-weighted frame pivoted to said firstmentioned frame to rotate about an axis at right angles to the axis of said first frame, a detector elementsuspended from, and disposed within. the said second frame, and leads extending through said casing and in electrical contact with said detector element.

2. In a drill hole magnetometer apparatus the combination of a non-magnetic', watertight cas ing adapted for insertion into a drill hole; a hollow member of insulating material removably secured within the casing, gimbals supporting said hollow member in a vertical position irrespective of the axial dispositionof the casing; a saturaFJ tion induction detector element; a flexible inem= ber suspending the said detector element within the hollow member; and leads electrically connected to the detector element, said leads extend ing outwardly of the said casing.

3. In a drill hole magnetometer apparatus the combination ci a substantially cylindrical, non magnetic, watertight casing adapted for insertion into a drill hole; an eccentrically pivoted frame oi insulating material rotatably mounted within the casing; a second eccentrically pivoted frame of insulating material carried by the first frame for rotation about an axis normal to that of: the said iirst frame; a saturation induction detector element suspended from the said second frame; and leads extending through one end of the casing and electrically connected to the said detector element.

.In a drill hole magnetometer apparatus, `la substantially cylindrical pressure-resistant casing adapted to be inserted in a drill hole, va nonmagnetic cradle pivoted on an axis coincident with the axis of said casing, weights carried'by said cradle and eccentric to the pivotal axis thereof to swing the cradle to assume an equilibrium position under the influence of gravity, a nonmagnetic sleeve pivotally supported by said cradle to swing about an axis at right angles to the aXis of said cradle, a weighted element carried by said sleeve to swing same to assume a vertical equilibrium position under the inuence of gravity, stop means to limit the rotation of said sleeve, a saturation induction coil suspended within said sleeve, and means to enable said coil to be balanced to assume a vertical position within said sleeve.

5. A device as claimed in claim 4 in which the pivots of said cradle `are electrical conductors, and cable means are provided extending into said casing to `connect a remote station with said pivots, said coil having the ends thereof connected with pigtail windings to provide electrical connection with said pivots and to permit the rotation oi said sleeve and coil.

6. Apparatus for exploring the magnetic character of subterranean formati-ons comprising a water-tight casing of non-magnetic material adapted for insertion into a drill hole; a saturable core within the casing; means orienting the core in a vertical position irrespective of the inclination of the casing; a coil carried by the core; an oscillator providing a sinusoidal current of predetermined frequency and sufficient to saturate the said core; a filter network connected between the oscillator and the coil, said iilter network adjusted to the oscillator frequency; a receiver responsive to the E. M. F. induced in said coil; and a lter network connected between the receiver and said coil, said latter filter network adjusted to pass only the second harmonic of E. M. F. induced in the said coil.

7. The invention as recited in claim 6 wherein the means orienting the core in a vertical position comprises an eccentrically-weighted frame pivotally mounted in the casing, a second eccentrically-weighted frame pivotally mounted within the first frame for rotation about an axis :normal to that of the said rst frame, and a resilient member suspending the said core within the said second frame.

S. The invention as recited in claim 7, in combination with a source of D. C. potential, means to adjust the magnitude of the D. C. potential to :balance out the induced voltage in the said coil by the earths magnetic field, and means blocking D. C. potentials induced in the coil from the said receiver.

9. A method of investigating the magnetic character of subterranean formations consisting of running a drill hole through the formation, inserting a saturation induction element, including a core, into the drill hole to a known depth, applying a known magnitude of D. C. current into theI induction element to create a magnetic flux in its core to balance out the flux in the core arising from the earths normal magnetic eld, applying an A. C. current of predetermined frequency to the induction element, and measuring the magnitude of the second harmonic of the resulting A. C. voltage induced in the induction element solely by the vertical component of the magnetic lheld around the said element.

ARTHUR A. BRANT.

REFERENCES CITED The following references are of record in the nie of this patent:

UNITED STATES PATENTS Number Name Date 2,338,991 Arnold Jan. 11, 1944 2,441,065 Green May 4 1948 OTHER REFERENCES Geophysics, July 1946, pages 321-334. Geophysics, August 1948, pages 182-214. 

